1. Field of the Invention
The present invention relates to an inverter device for an automobile which generates an AC output from a battery loaded on a vehicle body to drive a motor.
2. Description of the Related Art
Recently, an air conditioner equipped with an electrically driven compressor driven by a battery power source has been developed as an air conditioner for an electric automobile. This air conditioner comprises a battery (DC power source), an inverter device and an electrically driven compressor.
The above-mentioned inverter device comprises a switch as a switching device connected in serial to the battery; a charging device connected in parallel to this switch and including a resistor and a switch; an inverter module configured by molding, into a mold package, a switching element group including a switching element and an unshown diode to absorb a switching surge; a condenser; etc. The switching element group of the inverter module converts a DC voltage from the battery into a three-phase AC voltage, and applies it to the electrically driven compressor to drive a motor of the electrically driven compressor.
The above-mentioned condenser stably supplies a voltage to the switching element group. The resistor of the charging device restricts an incoming current through the condenser and an incoming current generated in the condenser when the DC voltage of the battery is applied. That is, the presence of the resistor makes it possible to, when the battery is connected, open the switch, close the switch of the charging device, and pass a current via the resistor, thereby restricting the incoming current produced when the voltage of the battery is applied. This makes it possible to prevent such a disadvantage that a high current or a high voltage is applied to the inverter module to damage the switching element group and the like in the inverter module.
In such an inverter device, a ground fault might occur, for example, in a positive line and a negative line of the battery, or inside the motor of the electrically driven compressor. Especially when the ground fault has occurred in the negative line, a leak current does not run despite the ground fault because a high voltage side is usually insulated, which has not allowed the ground fault to be detected. Thus, if the ground fault is left as it is without being noticed and if a further ground fault occurs at another place, a leak current might run to result in an electric shock, or a fire or damage of equipment due to the current.
Furthermore, means to detect such a ground fault has also been developed which is provided with an oscillating circuit to form a predetermined pulse waveform, and a ground fault detection circuit including a detection circuit to detect the pulse waveform from the oscillating circuit, thereby detecting the occurrence of the ground fault from a change in the pulse waveform of the oscillating circuit (e.g., refer to Japanese Patent Publication Laid-open No. 2933490).
However, when the ground fault detection circuit as mentioned above is provided, the circuit to detect the ground fault is complicated, leading to a problem of an increased production cost.